The key idea of #"ligand field theory"# is that a #"ligand field"# or #"crystal field"# imposed by 6 ligands coordinating to a cationic metal centre in an octahedral array, STRONGLY differentiates between the axial #"d-orbitals"#, and the #"non-axial orbitals....."#
Because the non-axial orbitals are offset from the six vertices of the octahedron, these orbitals, #underbrace(d_"xz", d_"yz", d_"xy")_(t_2g" set")# are somewhat stabilized with respect to the axial orbitals, #underbrace(d_(x^2-y^2),d_(z^2))_(e_g" set")#....depending on the electronic configuration, #d^4#, #d^5#, #d^6#, and #d^(7)# electronic configurations can be either #"LOW SPIN (electrons paired)"#, or #"HIGH SPIN (electrons unpaired)"#, in the resultant transition metal complex. And thus a simple measurement of magnetic moment, can probe the electronic structure of the metal complex.
Low spin and high spin configurations of a #d^4# metal complex are shown. You will have to read your text for the other configurations.....
